The invention relates to the use of halftone threshold arrays on digital output devices, and more particularly to threshold arrays for increased levels per color component.
A continuous tone image does not print well on most printing devices, so the image is usually printed as pattern of dots based on a grid. The grid generally consists of an array of halftone cells, each of which represents one section of continuous tone in the original image. When reproducing a halftoned image using a digital recording device, a halftone cell consists of a number of device pixels. In a device that produces only black and white pixels, some of the display pixels of each halftone cell are turned black to form dots that are relatively larger or smaller to represent darker or lighter portions of the original continuous tone image. In a dark halftone cell, most of the pixels are black, while in a light halftone cell, most of the pixels are white. A complete grid of the original image is composed of many such halftone cells, each of which has an independent density of displayed pixels and therefore a different apparent darkness when viewed from a distance.
A conventional method of selecting which dots to turn black works as follows. For a given halftone cell, the original image is sampled at each display pixel location in the halftone cell to obtain a sample value. This sample value is represented digitally as a number in a fixed range, typically zero to 255. The sample value is then compared to a threshold value at the display pixel location and the display pixel is turned white if the sample value is greater than the threshold value, and black otherwise. The threshold values, in turn, are supplied by a threshold array, which provides a threshold value for each pixel in the halftone cell. This process is carried out for each halftone cell of the image.
The term threshold array is commonly used to denote a set of generic pixels, each of which has a "threshold value". The device plane is tiled with copies of the threshold array, so that each device pixel is mapped to one generic pixel and its threshold value. After the desired sample value for the device pixel is computed--from the image being halftoned, for example--it is compared to the threshold value. If the sample level is greater than the threshold value, the device pixel is left white; otherwise, it is marked black.
A threshold array is much like a sampled image: it is generally a rectangular array of pixel values defined entirely in device space. It can be built algorithmically, by use of a spot function, for example, or it can be built by hand. The sample values occupy some number of bits: in a typical system, the sample values occupy 8 bits that represent gray levels ranging from zero for black and 255 for white.
The scheme generalizes to monochrome devices with multiple bits per pixel. For example, with 2 bits per device pixel, each device pixel can directly represent one of four different gray sample levels. For each device pixel that is to be painted with some in-between gray level, the corresponding pixel of the threshold array is consulted to determine whether to use next-lower or next-higher directly-representable gray level. In this situation, the samples of the threshold array do not represent absolute gray values, but rather gradations between two adjacent representable gray levels.
Halftoning is also used to approximate continuous-tone colors by a pattern of pixels that can achieve only a limited number of discrete colors. The input to the halftone function includes continuous-tone color components in the device's native color space. The output includes pixels representing colors the device can reproduce. Some devices can reproduce continuous-tone colors directly. These are known as "contone" devices. For such devices, halftoning is not required and color components can be transmitted directly to the marking engine of the device.
A halftone defined in this way can also be used with color output devices whose pixels consist of component colors that are either completely on or completely off. Most color printers, but not color displays, work this way. Halftoning is applied to each color component independently, producing shades of that color. The red, green, and blue values, for example, are created independently as gray levels, and a threshold array is applied to each color.
Further information on halftoning may be found in U.S. Pat. Nos. 5,285,291 and 5,305,118, as well as in such standard reference works as Foley, van Dam et al., Computer Graphics, Addison-Wesley (2d ed. 1992), pp. 568-573, and Adobe Systems Incorporated, Adobe PostScript.RTM. Language Reference Manual, Addison-Wesley (2d ed. .COPYRGT.1990), pp. 309-319.